This invention is in the field of semiconductor solar cells, and more particularly is a semiconductor solar cell with an inversion layer having a relatively high concentration of charge carriers and a method of preparation thereof.
Semiconductor solar cells are well-known in the art. They are known to have p-n junctions near the surface of the device which are exposed to solar light radiation. The photoelectric energy is created by light waves which generates hole-electron pairs. Solar cells, such as Si or CdS cells are sensitive to wave lengths from about 1 .mu. down to 0.45 .mu..
It is well-known that a silicon dioxide layer formed on the surface of p-type silicon or other p-type semiconductors will produce an n-type inversion layer in the semiconductor. Thus, a p-n junction will exist at a shallow depth. However, it is widely accepted that the ordinary oxidation of p-type silicon surface results in an inversion layer having a charge concentration of only 10.sup.11 charge carriers/cm.sup.2 as contrasted with a diffused type junction with a diffused layer having a charge concentration of 10.sup.15 carriers/cm.sup. 2. It may be noted that an inversion layer is induced and when the inducing material, e.g., silicon dioxide, is removed the inversion layer disappears and a substrate reverts back to the p-type semiconductor.
The reason why the silicon dioxide produces an inversion layer in the semiconductor material is attributed partially to interface states and also to the fact that silicon dioxide has a tendency to become positively charged when in contact with another material.
Oxidized silicon surfaces have been studied in connection with transistors, micro circuits and MOS transistors. All these studies indicate that the oxide induces in silicon a negative charge having a charge concentration of approximately 10.sup.11 charge carriers/cm. The oxide may also be contaminated with foreign ions such as hydrogen or sodium, etc. These ions are usually positive so that they can add to the produced charge and raise it to 10.sup.12 charges/cm.sup.2. Generally, the presence of foreign ions is undesirable because their density cannot be controlled and they are mobile in the oxide.
U.S. Pat. No. 3,769,558 of Lindmayer discloses a semiconductor device of a p-type substrate having oxide covering layers of silicon and chromium of thicknesses between 800 and 1,000 angstroms thick. The oxide covering gives rise to inversion layer of the n-type conductivity on the bulk substrate. The collection of the charge to the external circuit would be accomplished by a Schottky type metal-semiconductor contact wherein the metal is in direct contact with the semiconductor.
One of the concepts proposed for obtaining electrical energy is the electrical power from space concept. This concept involves satellites in space equipped with solar cells which would convert the solar energy to DC power. The DC power generated in space by the satellites would be beamed by means of microwaves to the ground and reconverted to high voltage DC or AC power. The essential characteristics of the solar cells for use in this application are that the solar cells be capable of covering as much of the solar spectrum as possible to obtain the highest practical conversion efficiency and that the fabrication of the solar cells be free of critical processing steps so as to reduce the cost of producing them and to make them feasible for large-scale use.